These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: Kingdom Hearts: Chain of Memories
Breather Level: Like in Kingdom Hearts, The 100 Acre Wood fills this role, with no enemies and a trove of new, free cards and sleight combinations.
The Duel system in Re:Chain of Memories, especially Barrage. This attack will always hit and always do 1 bar damage. It's possible to beat Ansem easily without Dark Mode because of this.
Certain sleights can make the game very easy, if you build your deck right. With enough 0s, you can't be cancelled, and a ton of Sonic Blade + Elixir = Win. Once you have the Jafar card this makes using sleights like Sonic Blade even more deadly. A deadly pair to use would be Jafar with Ars Arcanum. Normally this sleight would require 3 cards from 1-6 in value, very breakable. If Jafar is used before, the sleight becomes unbreakable. This makes most bosses become a breeze.
The Tornado Sleight (Aero + Gravity + Summon card) is absolutely unfair in both versions of the game: it creates a large funnel cloud that the player can control, which sucks up and immobilizes all Heartless it touches while dealing massive damage.
The Oogie Boogie card (Regen) quickly becomes one of the best buff cards, especially against tough bosses.
The Lethal Frame (mistranslated as "Lethal Flame") sleight (Stop + Attack + Attack) stops time and allows Sora to quickly deliver a bunch of hits. When time starts again, the enemy starts taking damage and can't act for a few moments, allowing you to set up another one. Long strings of these can deal massive damage in boss fights.
Combined with Awesome yet Practical for bosses: Simply using 0's to Card Break your opponent's deck. Every card you break with 0 can't be reshuffled by the boss, while you can recharge your deck indefinitely. After awhile, the boss will simply begin to run out of cards to make combos with, making him easy prey.
Genius Bonus: In the manga adaptation of COM, Larxene is seen reading a book about French writer Marquis De Sade. We'll let you do the math.
Any enemy you have to attack from behind in the Game Boy Advance game. This is remedied a bit in Re:CoM because you're able to attack from more angles - in the original, it's roughly 2D so you have to be right behind them. Defenders are easily the most annoying mooks in the game.
The Creeper Plants have a terrible tendency to break your cards.
Neoshadows. They also have a tendency to turn two-dimensional and avoid everything you throw at them. This is especially annoying because they seem to wait right until you use a sleight.
Good Bad Bugs: The "Double jump glitch" in the Game Boy Advance version. When you double-jump (B twice) and perform an attack, Riku will jump down at his enemy and hit them numerous times... but if you have an attack modifier like Overdrive, Berserk, or Attack Bracer on, it doesn't take down the charges and this attack still has the effects of the modifier.
Vexen's shockingly brutal death was mitigated somewhat by the "Can't feel real emotions" tidbit from Kingdom Hearts II, but When 3d reveals nobodies regrow hearts and can experience emotions, it becomes just as bad if not worse than it first was, because Vexen was legitimately terrified at the thought of dying.
Hilarious in Hindsight: Larxene's voice actor, Shanelle Gray, also narrates some of the Wendy's commercials. Then, during the cutscene just before fighting her a second time,
Larxene: Okay... Have it your way. More pain for you means more fun for me!
When one considers Zexion's life, from his childhood in Birth by Sleep to when he meets his end in Chain of Memories, he sorta becomes this.
Vexen and Larxene also count, mainly due to their deaths, which are played tragically. Larxene gets bonus points in the Japanese version due to being voiced by Yuko Miyamura.
Marluxia may qualify in hindsight, since he and Larxene's whole reason for their rebellious plot was that they figured out that Xemnas was in fact using the other Organization members and never intended on them getting their hearts back at all. It was just bad circumstance that Saix had the same notion to betray Xemnas but wanted it on his own terms, so he had Axel ensure Marluxia's downfall.
Love to Hate: Marluxia, Larxene, and Axel are so deliciously evil.
The final battle against the Riku Replica (in both Sora AND Riku's modes), so freaking much.
Ursula in Reverse/Rebirth. She may actually be harder than the Final Boss.
Any time you fight Vexen in the original Chain of Memories. You have to attack him from behind - this is harder in the original than it is in the remake. He also has a tendency to turn right around and attack you when you try to hit him.
Vexen also has a one-time use Enemy-type card that, when HP goes to 0, revives him with a quarter of his HP restored. Yeah, you thought you were finally finished with him, didn't you? Thankfully, it's also his drop loot, so you can acquire and use it afterward.
That isn't to say that Re:CoM Vexen is easy. His attacks are pretty hard to dodge and can stunlock, he still can't be attacked from the front and his deck is stacked with high-numbered attack-cards. He also has Elixirs and Hi-Ethers with him, making the strategy of breaking all his good cards hard to execute - especially in the rematch, in which he also owns a card that prevents breaking items.
Captain Hook. His Geo Effects screw up your summon cards, his dodge rate is insane, he can throw bombs seemingly without end, and his sword attack is extremely difficult to dodge. Better bring your zeroes and your cures - you're gonna NEED it all. This review says it all:
You may think that all the bosses before Captain Hook might have been annoying but otherwise easy. You might have thought that the rest of the game would go along like this. Even if that wasn't the case, even if you thought that there would be at least one tough boss along the way, nothing could prepare you for the brokenness that is Captain Hook.
Hades is a good example of this trope as well. He is one of the only Disney bosses that uses sleights, and loves making your life a living hell with them, which hit like a truck.
What an Idiot: All Jiminy writes in his Journal at the end of Chain of Memories is "Thank Naminé" with no explanation whatsoever. Cue the mass confusion for Sora & co. at the beginning of II. Considering that this was fairly important to them, you'd think the note would be a bit more thorough.
The Woobie: Naminé, Kairi's Nobody, who is confined by the Organization to the top of Castle Oblivion for the most part, is forced to do their bidding, and is perfectly aware of her status as a temporary being.